A field effect transistor structure which is formed with respect to an n-channel field effect transistor (n-FET) is illustrated schematically in cross section in FIG. 11. The n-FET comprises an active zone formed in a semiconductor substrate 1 and also a gate structure 2 bearing on a structure surface 10 of the semiconductor substrate 1. In the active zone, two source/drain regions 61, 62 are spaced apart by a channel region 63. The source/drain regions 61, 62 are n-doped sections of the semiconductor substrate 1 and adjoin the structure surface 10 of the semiconductor substrate 1. The channel region 63 is intrinsically conducting or p-doped. The gate structure 2 comprises a gate electrode 26, which is arranged over the channel region 63 above the structure surface 10 and is insulated from the semiconductor substrate 1 by a gate dielectric 20. Furthermore, the gate structure 2 has spacer structures 24, which are arranged along vertical sidewalls of the gate electrode 26 with respect to the structure surface 10 and in each case bear on the structure surface 10 in a manner oriented toward one of the source/drain regions 61, 62.
In the example illustrated, the source/drain regions 61, 62 result in each case from the superimposition of an n-doped basic section 12n with an n-doped extension section 11n. In a manner governed by the fabrication, the boundary edges of the basic sections 12n which are in each case oriented toward the channel region 63 are essentially aligned with the outer edges of the spacer structures 24. The boundary edges of the extension sections 11n which are oriented toward the channel region 63 are essentially aligned with the outer edges of the gate electrode 26 and, in the example shown, essentially adjoin a section of the channel region 63 which can be controlled by a potential at the gate electrode 26. The extension sections 11n form a low-resistance coupling of a channel formed in the region of the gate dielectric 20 in the conducting state of the n-FET to the respective source/drain region 61, 62.
The extension sections 11n and basic sections 12n are in each case produced by ion implantation.
The boundary edges essentially define the originally implanted region. Within the implanted region, the dopant concentration is essentially uniform directly after the implantation. Diffusion of the dopant results in gradual transitions at the boundary edges of the individual doped sections.
In the off state of the n-FET, the two source/drain regions 61, 62 are electrically insulated from one another. When a suitable potential is applied to the gate electrode 26, mobile electrons are accumulated in a section of the channel region 63 which adjoins the gate dielectric 20, and they form a conductive channel between the two source/drain regions 61, 62. The n-FET then begins to conduct.
Due to the dopant gradient as well, the extension sections 11n undercut the gate electrode 26. As the respective source/drain region 61, 62 increasingly overlaps the gate electrode 26, a parasitic overlap capacitance between the respective source/drain region 61, 62 and the gate electrode 26 increases. An increased parasitic capacitance delays the switching of the n-FET between conducting and blocking state and consequently increases the switching losses.
The parasitic capacitance between the gate electrode 26 and the respective source/drain region 61, 62 is reduced by the inner edge—oriented toward the channel region 63—of the respective source/drain region 61, 62 or of the respective extension section 11n being drawn back outward from the channel region 63. The connection of the comparatively heavily doped basic section 12n of the respective source/drain region 61, 62 to the section of the channel region 63 which can be influenced by a potential at the gate electrode 26 is then formed by a comparatively weakly doped section and the nonreactive resistance of the coupling of the conductive channel to the respective source/drain region 61, 62 is high.
A reduction of the overlap capacitance between the gate electrode 26 and the respective source/drain region 61, 62 is accompanied by an increased nonreactive resistance of the channel coupling between the region controlled by the gate electrode 26 and the respective source/drain region 61, 62.
In order to optimize the overlap capacitance relative to the resistance of the channel coupling, the overlap between the extension section 11n and the gate electrode 26 is chosen such that the doping outside the section which can be influenced by an electric field at the gate electrode 26 is high enough to ensure a sufficiently low-resistance channel coupling in the conducting state.
As the slope of the dopant gradient increases, the extension section can be drawn back further from the channel region with a consistently low nonreactive lead resistance.
The accompanying reduction of the overlap capacitance is partly compensated for, however, by the simultaneously required reduction of the thickness of the gate dielectric in the transition to smaller feature sizes and greater slopes in the dopant gradient.